This invention is directed to a process for the preparation of sulfur-modified chloroprene polymers which does not require a separate peptization step.
Sulfur-modified chloroprene polymers are well known and are available commercially under the trade designation Neoprene G types. They are made by polymerization of chloroprene or copolymerization of chloroprene with another unsaturated organic monomer in the presence of elemental sulfur. The polymer, as made, contains sulfur atoms in its chain, and at high conversion it is appreciably crosslinked. Because of its normally high molecular weight, this polymer cannot be processed directly but must first be broken down to a lower molecular weight. This step, known as peptization, is normally accomplished by treating polymer latex with a tetraalkythiuram disulfide in combination with a sodium dialkyl dithiocarbamate or other sulfur-containing nucleophile. Preparation and peptization of sulfur-modified chloroprene polymers is described, for example, in the following U.S. Pat. Nos. 2,234,215 (Youker); 3,595,847 (Mayer-Mader); and 3,920,623 (Kahn).
The peptization step, while necessary in the commercial processes for preparing Neoprene G types, is regarded by the industry as a nuisance since it is time-consuming (frequently requiring several hours) and ties up valuable plant space for holding tanks and associated equipment. Because of this equipment, the peptization step also requires additional capital investment.
Peptization of sulfur-modified chloroprene polymers can be readily accomplished by use of tetraalkylthiuram disulfides in the presence of sulfur-containing nucleophiles. These compounds have not been heretofore used in the polymerization step itself because they often inhibit polmerization. While the possibility of carrying out the polymerization in the presence of a tetraalkylthiuram disulfide was suggested in U.S. Pat. No. 1,950,439 to Carothers et al. as early as 1934, the disadvantages of running the polymerization in the presence of tetraalkylthiuram disulfides were always considered to outweigh any possible advantages. Yet, because tetraalkylthiuram disulfides are readily available and well accepted by the industry, it would be desirable to be able to conduct the chloroprene/sulfur copolymerization in their presence under such conditions that a highly crosslinked polymer would not be formed and a separate peptization step prior to isolation would not be necessary.